Saving money for a rainy day isn’t just a good idea in New Jersey — it’s about to become the law.

Democratic Gov. Phil Murphy is poised to sign a “rain tax” bill passed by the state legislature Jan. 31 — and Republicans and lots of taxpayers are howling with rage.

“Every time you think there’s nothing left to tax, we come up with something else,” Assemblyman Hal Wirths (R-Morris-Sussex) exploded during a debate on the measure.

“It’s just never-ending down here.”

The law allows each of the state’s 565 municipalities to set up its own public stormwater utility. The new bureaucracies will build and manage sewer systems to treat pollutant-filled stormwater runoff.

The infrastructure could cost billions, state authorities say. Under the law, the utilities can levy steep fees on properties with large parking lots, long driveways, or big buildings — which create the most runoff.

The state would scoop up 5 percent of the proceeds.

The idea for the new fee goes back to 2010, when President Obama’s Environmental Protection Agency ordered states whose rivers and streams flow into the Chesapeake Bay to drastically cut sediment pollution.

Maryland was the first to fall into line, with a 2012 law that charged cleanup costs to property owners — and sparked taxpayer fury. Republican Larry Hogan’s promise to repeal the “rain tax” helped sweep him into the governor’s mansion in 2014.